As someone with tens of thousands invested into index funds: The average growth in the stock market for the past few hundred years has been about 7% a year, so while I’m undecided on the fact wether it can grow infinitely, what ever that means, I’m pretty certain it’ll keep doing so throught my lifetime, so I consider it relatively safe place to put my money into. Atleast not less safe than most of the alternatives. Also, in the event of total stock market crash where everything effectively goes permanently to zero, then something so big is going on, that this is probably the least of anyone’s problems and it doesn’t matter where your money is unless it’s in food and water.